ifb: Include <linux/sched.h>

commit b51c3427 ('ifb: fix rcu_sched self-detected stalls', commit
440d57bc upstream) added a call to cond_resched(), which is
declared in '#include <linux/sched.h>'.  In Linux 3.2.y that header is
included indirectly in some but not all configurations, so add a
direct #include.
Reported-by: Teck Choon Giam <giamteckchoon@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/net/ifb.c

@@ -34,6 +34,7 @@
 #include <linux/init.h>
 #include <linux/interrupt.h>
 #include <linux/moduleparam.h>
+#include <linux/sched.h>
 #include <net/pkt_sched.h>
 #include <net/net_namespace.h>